Recently, in order to maximize the performance of the wireless communication system and the communication capacity, Multiple Input Multiple Output (MIMO) systems have been drawing a great deal of attention. The MIMO technology corresponds to an evolved version of the conventional communication technology using a single transmission antenna and a single reception antenna, and the MIMO technology applies multiple transmission antennas and multiple reception antennas, so as to enhance the transmitted and/or received (or transceived) data transmission efficiency. Herein, a MIMO system may also be referred to as a multiple antenna system. The MIMO technology applies the technology of receiving a plurality of segmented data fragments being transmitted through multiple antennas and completing the received message by grouping the collected data fragments, instead of relying on a single antenna path, in order to receive a single full message. As a result, the data transmission rate may be enhanced within a predetermined range, or a system range may be increased with respect to a specific data transmission rate.
The MIMO technology may include transmit diversity, spatial multiplexing, and beamforming. Herein, transmit diversity corresponds to a technology of transmitting the same type of data through multiple transmission antennas, so as to enhance the transmission reliability. Also, spatial multiplexing corresponds to a technology of having different types of data being transmitted at the same time through multiple antennas, thereby being capable of transmitting data at a fast transmission rate, without having to increase the system bandwidth. Moreover, beamforming is used for increasing an SINR (Signal to Interference, plus Noise Ratio) of a signal by adding a weight respective to a channel state (or status) in a multiple antennas system. At this point, the weight may be expressed as a weight vector or a weight matrix, and this may also be referred to as a precoding vector or a precoding matrix.
Also, spatial multiplexing may include spatial multiplexing respective to a single user and spatial multiplexing respective to a plurality of users. Accordingly, spatial multiplexing may also be referred to as a single user MIMO, and spatial multiplexing respective to a plurality of users may also be referred to as SDMA (Spatial Division Multiple Access) or Multi User MIMO.
The capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel may be separated (or divided) to independent channels. When the number of transmission antennas is referred to as Nt, and when the number of reception antennas is referred to as Nr, the number of independent channels Ni becomes Ni=min{Nt, Nr}. Each of the independent channels may be referred to as a spatial layer. As a non-zero eigenvalue of a MIMO channel matrix, a rank may be defined by a number of spatial streams that can be multiplexed.
As shown in the example of a single user MIMO shown in FIG. 1, the single user MIMO corresponds to a structure wherein multiple data streams, each being different from one another, transmitted from the base station are all transmitted to a single user. In case of the single user MIMO, a MIMO channel consists of one transmitter and one receiver. In case of the single user MIMO, one user may receive all of the transmitted signals. Therefore, in case of the single user MIMO, only the data respective to a single user are scheduled to the same time/frequency domain(s). Conversely, as shown in the example of a multiple-user MIMO shown in FIG. 2, the multiple-user MIMO respectively transmits the multiple data streams, each being different from one another and being transmitted from the base station, to the plurality of users. In case of the multiple-user MIMO, one transmitter and multiple receivers collectively configure the MIMO channel. Therefore, in case of the multiple-user MIMO, the data respective to the plurality of users may be collectively scheduled to the same time/frequency domain(s).
In order to allow the user equipment to demodulate the data allocated (or assigned) to a predetermined time/frequency domain, the user equipment uses a reference signal (RS), which is transmitted from the base station, so as to perform a channel estimate (or channel estimation) for estimating the configuration and channel quality of a physical channel, which is used for the data transmission. The method for estimating a channel and the reference signal will hereinafter be described in detail. In order to detect a synchronization signal, the receiver is required to be aware (or informed) of information (e.g., attenuation, phase deviation, time delay, and so on) on a radio channel. At this point, channel estimation refers to a process of estimating the size and reference phase of a carrier (or carrier wave). A radio channel environment has a fading characteristic, wherein a channel status chronologically and irregularly changes within time and frequency domains. Herein, the process of estimating an amplitude and phase respective to such channel is referred to as channel estimation. More specifically, channel estimation refers to a process of estimating a frequency response of a radio section or radio channel. Herein, the method for performing channel estimation includes a method of estimating a reference value based upon the reference signal of several base stations by using a two-dimensional channel estimator. At this point, a reference signal refers to a symbol not including any actual data, in order to support (or help) the processes of carrier phase synchronization and base station information acquisition (or reception), yet yielding a high output. A transmitting end and a receiving end may perform channel estimation by using the above-described reference signal. The channel estimation process, which is performed by using the above-described reference signal, refers to a process of estimating a channel by using a reference signal, which is commonly known by the transmitting end and the receiving end, and of recovering data by using the estimated value.
In the multiple-user MIMO, in order to demodulate the transmission data, each of the user equipment is required to be capable of using the acquired channel information, which is received from the base station through the reference signal, so as to separate (or divide) each transmission layer, which is used for transmitting data to each of the corresponding user equipment. In order to do so, the base station is required to signal reference signal allocation information respective to each reference signal being allocated to each of the corresponding user equipment.